Carbonylation has been widely employed for various chemical processes, for example, synthesizing a carboxylic acid having n+1 carbon atoms by the reaction of an alcohol having n carbon atoms with carbon monoxide in the presence of catalysts.
U.S. Pat. No. 3,769,329 discloses a rhodium catalyst system for preparing acetic acid by carbonylating from methanol at relatively lower temperature and pressure, for example, at a temperature less than 300° C. and at a partial pressure of carbon monoxide lower than 15,000 psig. U.S. Pat. No. 4,690,912 further discloses that catalyst selectivity, activity and stability in carbonylation can be improved by a halogen-containing promoter component, while the carbonylation is carried out at a relatively lower temperature and pressure. Product isolation and catalyst recovery are also improved when compared to the prior art.
Although the rhodium catalyst systems taught in the above-mentioned patents can catalyze carbonylation at a lower temperature and pressure, and have the advantage of high reaction selectivity, insoluble rhodium (III) species are formed and precipitated out when methanol is carbonylated to acetic acid in the presence of rhodium catalysts at a low partial pressure and a low level of water. Therefore, the process should be operated in the presence of higher water content to maintain catalyst activity and high reaction rate. Such a high water-containing reaction system, however, can lead to large energy consumption in the subsequent purification of acetic acid.
To prevent rhodium catalyst precipitation, EP 0055618 teaches that an organic compound containing one or more nitrogen atoms, phosphorus atoms or —COOH can be added as a catalyst stabilizer. U.S. Pat. No. 4,733,006 further teaches that alkali metal acetates, such as lithium acetate, can be used as a stabilizer to overcome rhodium catalyst precipitation. Nevertheless, the relevance of a catalyst stabilizer to reaction rate of methanol carbonylation is not discussed in the aforesaid patents.
It is disclosed in U.S. Pat. No. 5,001,259 that rhodium catalyst precipitation can be improved and substantially the same reaction rate as those proceeded in high water content can be attained, when 10-20 wt % of an iodide salt of group IA metals or a group IIA metals, or a quaternary ammonium iodide is used as a rhodium catalyst stabilizer in methanol carbonylation at 1-4 wt % of water content. However, an insoluble soluble complex of rhodium with quaternary ammonium iodide (N-methylpicolinium iodide) is easily formed and precipitated from the reaction solution. As a result, the consumption of the rhodium catalyst is undesirably increased. Additionally, U.S. Pat. No. 5,001,259 indicates that high methyl acetate concentration can poison rhodium the catalyst stability even when a large amount of water is present.
Other stabilizers are described in EP153834. It discloses that thiols and imidazoles can be used as stabilizers to stabilize rhodium catalysts in the carbonylation. U.S. Pat. No. 5,442,107 further discloses that several heterocyclic nitrogen compounds are used as catalyst stabilizers in methanol carbonylation in the presence of low water content, which includes 2-ethyl-4-methylimidazole, 4-methylimidazole, 4-tert-butylpyridine, 2-hydroxylpyridine, 3-hydroxylpyridine and 4-hydroxylpyridine. However, the above-mentioned prior art fail to teach how a reaction rate is dependent on a catalyst stabilizer at low water content. Meanwhile, the disclosed catalyst stabilizers also easily react with rhodium to form insoluble complex that would precipitate from the reaction solution.
Therefore, it is desired to develop a method for preparing carboxylic acid such that a rhodium catalyst can be effectively stabilized under strict carbonylation conditions to eliminate catalyst precipitation and to keep high reaction rate, and further to reduce the energy consumption for separation and purification of acetic acid product.